This application is based on Japanese Patent Application No. 11-258800 filed on Sep. 13, 1999, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a method and an apparatus of an injection molding, which are arranged to successively mold articles made of a resin material. More particularly, the present invention is concerned with a novel method and a novel apparatus of an injection molding, which assure a sufficiently high degree of stability of a heat history of the resin material and a temperature of a mold, in every molding cycle, while permitting an execution of a successive injection molding of the resin material with high stability.
2. Description of the Related Art
As one type of an injection molding machine, there is known an injection molding machine, such as an inline-screw-type injection molding machine, a plunger-type (torpedo-type) injection molding machine and a preplasticating injection molding machine, wherein an injection screw or plunger which is reciprocally movably disposed in a bore of a heating cylinder is rotated so that a resin material is heated to be molten and metered in the heating cylinder. Successively, the injection screw or plunger is moved so as to inject the resin material into a mold cavity of a mold which is clamped by a suitable clamping device. The resin material filling the mold cavity is held within the cavity under a predetermined holding pressure and then cooled for a predetermined period of time. Then, the mold is opened and the molded product is ejected or released from the mold cavity, thereby obtaining a desired molded product. Such an injection molding machine is operated to perform repeatedly a cycle of molding operations so as to successively form a multiplicity of desired products.
In order to produce successively the desired products with a desired good quality, by means of the injection molding machine, there is required that a molding condition is kept substantially constant in every molding cycle. In particular, it is important that a heat history of the resin material and a temperature of the mold are kept substantially constant in every molding cycle.
However, one molding cycle includes various kinds of steps such as a cooling step wherein a time of the step is controlled, a mold closing and opening step wherein an operation velocity of the injection molding machine is controlled by adjusting an amount of discharge of a hydraulic pump, and an injection step wherein an injection pressure or a position of the injection screw are controlled. Therefore, it is difficult to keep the injection molding condition substantially constant, and a conventional injection molding machine inevitably is likely to suffer from variation in heating conditions and heating times of the resin material or the mold, leading to difficulty in keeping the heat history of the resin material and the temperature of the mold substantially constant in the every molding cycle, upon performing the above-indicated successive injection molding. Due to this drawback, the conventional injection molding machine may be insufficient in attaining a desired degree of stability in quality of the products, depending on kinds of the products.
It is therefore a first object of the present invention to provide a method of injection molding for successively producing a resin product by repeatedly performing an injection molding cycle, which method permits a high degree of stability of molding conditions in terms of a heat history of a resin material and a temperature of a mold, in every molding cycle, thereby permitting successive production of the resin products with a stabilized quality thereof.
It is a second object of the invention to provides an injection molding apparatus which is novel in construction and which permits that molding conditions such as a heat history of a resin material and a temperature of a mold are kept substantially constant, during its successive injection molding operation, thereby permitting a successive production of desired resin products with a stabilized quality thereof.
The first object may be achieved according to a first aspect of this invention, which provides a method of injection molding for repeatedly performing an injection molding cycle including the steps of closing and clamping a mold having a mold cavity, injecting a resin material into the mold cavity, cooling and solidifying said resin material filling the mold cavity, and opening the mold so as to remove a molded product, in order to successively form the molded product, the method comprising the steps of: (a) measuring a cycle time at at least one measuring point in the injection molding cycle to thereby obtain a measurement of the cycle time; (b) obtaining a difference between the measurement of the cycle time and a predetermined reference cycle time; and (c) executing a time compensation on the basis of the difference at a time controlled step which is arranged to be executed before the next measuring point comes.
In the injection molding method according to the first aspect of the invention, the entire time required to execute one injection molding cycle is suitably controlled to be made substantially constant, although the injection molding cycle includes steps which are not controlled based on time. Therefore, the present method is capable of restricting or reducing an amount of variation in the cycle time of each injection molding cycle, making it possible that a period of time required from a current injection action to a next injection action is kept substantially constant. Thus, the present method permits a substantially constant molding conditions in terms of the heat history of the resin material and the temperature of the mold, in the every molding cycle.
For the above reasons, the present injection molding method assures an improved stability in quality of the products formed by the successive injection molding, while effectively attaining a sufficiently reduced ratio of reject of the molded product and an improved yield of the molded product.
It is noted that the measuring point for measuring the cycle time may be previously fixed to a point or points in the injection molding cycle, but not particularly limited. For instance, the measuring point may preferably be set to any point in the cycle at which an electric signal is generated. In this case, the electric signal is advantageously utilized to measure the cycle time. More specifically, the measuring point is set to a point in the cycle at which the injection molding cycle changes its steps. Namely the point may preferably be an initiation or a completion point of a mold clamping operation, an initiation or a completion point of an injection operation, for example. In this respect, the electric signal may be an output signal of a positioning sensor like a limit switch, an output signal of a timer or a time counter, for example.
The reference cycle time may be set to a desired value by a manual operation. It is preferable that the cycle time is timed in every molding cycle and if a desired product is obtained, then the timed cycle time is set as the reference cycle time. It is noted that the reference cycle time may be suitably changed taken into account molding conditions or the like.
The time compensation based on the obtained difference between the measurement of the cycle time and the reference cycle time may be executed at any one or more of the time controlled step(s) in the injection molding cycle. In this respect, the time controlled step should be interpreted as a step wherein a time or period of the step is controlled. For an effective time compensation, the time compensation is executed as follows: In this case where an injection molding cycle includes a plurality of measuring points for measuring the cycle time, if the difference is obtained in a current measuring point, then the time compensation based on the obtained difference should be executed until the next measuring point comes. In this case where an injection molding cycle include one measuring point for measuring the cycle time, if the difference is obtained in the measuring point in the current cycle, then the time compensation based on the obtained difference is executed until the measuring point in the next cycle comes.
In one preferred form of the method of the present invention, the time controlled step at which the time compensation is executed, may be selected from the group consisting of (i) an injection delay time after the step of clamping the mold is completed and before the step of injecting the resin material is initiated, (ii) a cooling time after the step of injecting the resin material is completed and before the step of opening the mold is initiated, and (iii) an intermediate time after the step of opening the mold is completed and before the step of closing the mold is initiated.
In the above preferred form of the method, the time compensation can be effectively executed in the above-indicated time controlled steps (i), (ii), (iii), so that the difference between the measurement of the cycle time and the reference cycle time is effectively compensated or zeroed in these time controlled steps, while an adverse effect of the time compensation on the molded product is effectively minimized or avoided.
It is required that at least one measuring point is set in an optional point in the injection molding cycle. Preferably, a plurality of measuring points (e.g., two, more preferably three or more measuring points) are set in respective points in the cycle. In this case, the method is desirably arranged such that the differences between the measurement of the cycle time and the reference cycle time are detected at respective measuring points, and the time compensation based on the respectively detected differences are executed at the different time controlled steps. In this arrangement, the measurements of the cycle time and the time compensations are executed at the plurality of points in the injection molding cycle. This means that a control of the cycle time can be performed at each time controlled step, resulting in an improved accuracy of the control of the cycle time, and a resultant accuracy of control of the heat history of the resin material and the temperature of the mold. In this arrangement, moreover, an amount of compensation of time to be compensated in one cycle time is effectively divided with the plurality of time controlled steps. This means that an amount of compensation of time in one time compensation action is effectively reduced, thereby reducing adverse effect of the time compensation action on the molded product.
In this respect, it is noted that the time compensation based on the time difference between the measurement of the cycle time and the reference cycle time, which difference is detected in one measuring point, may be executed at one time controlled step in the cycle, or alternatively may be executed at the plurality of time controlled steps in the cycle. The latter case is effective to reduce an amount of compensation of time to be executed in each step, thereby reducing the adverse effect of the time compensation on the quality of the molded product.
In another preferred form of the method, there is determined an upper limit of an amount of compensation of time executed at one time controlled step. In this form of the method of the invention, the presence of the upper limit of the amount of compensation of time is effective to prevent excessively large amount of change in a period of the time controlled step, even in the case where an excessively large amount of difference between the measurement of the cycle time and the reference cycle time is detected in the measuring point, thereby eliminating or minimizing the adverse influence of the time compensation on the operation of the injection molding during the time controlled step. The upper limit of the amount of compensation of time is not particularly limited to the specific value, but may be suitably determined taken into account a tolerable amount of change in the operation time of the time controlled step, a required quality of the product, a degree of influence of the change of the operation time of the time controlled step on the quality of the product, an amount of difference between the measurement of the cycle time and the reference cycle time generated under a standard molding condition, and the like. In a general injection molding apparatus, the upper limit of the amount of the compensation of time is preferably set at about xc2x10.5 second, more preferably, at about xc2x10.2 second.
The second object indicated above may be achieved according to a second aspect of the present invention which provides an injection molding apparatus for repeatedly performing an injection molding cycle including the steps of closing and clamping a mold having a mold cavity, injecting a resin material into the mold cavity, cooling and solidifying the resin material filling the mold cavity, and opening the mold so as to remove a molded product, in order to successively form the molded product, the apparatus comprising: (a) a time counter adapted to count a cycle time at at least one measuring point in the injection molding cycle; (b) a calculator adapted to calculate a difference between the cycle time obtained by the time counter and a predetermined reference cycle time; and (c) a compensating means for executing a time compensation on the basis of the difference calculated by the calculator at a time controlled step which is arranged to be executed before the next measuring point comes.
In the injection molding apparatus according to the second aspect of the present invention, the above-described injection molding method according to the first aspect of the present invention is effectively performed. Therefore, the present apparatus is capable of reducing or eliminating an amount of variation in the cycle time of each injection molding cycle, due to the variation of the operation time of the steps which are not controlled based on time, thereby assuring a high stability of the heat history of the resin material and the temperature of the mold in every molding cycle. This arrangement makes it possible to successively form a desired product with a stabilized quality.
While the time counter, the calculator, and the compensation means may be effectively embodied by utilizing a microcomputer, for example. Such a microcomputer may be a microcomputer used as a controller for controlling injection molding operations of the injection molding apparatus.
In one preferred form of the apparatus of the present invention, the apparatus further comprises a reference cycle time setting means for storing and renewing a measurement of the cycle time as the reference cycle time in a suitable memory device based on an inputted signal applied from an external area.
In this case where the present injection molding apparatus is constituted by utilizing a microprocessor, the actual measurement of the cycle time may be stored in a memory device of the microprocessor as the reference cycle time, based on an input signal applied to an input portion of the microprocessor from an external area. The stored cycle time may be renewed also based on the input signal applied to the input portion. This arrangement permits an easy and rapid setting or adjustment of the reference cycle time. For instance a cycle time which is measured when the injection molding apparatus produces a desired product, can be easily set as the reference cycle time, facilitating setting and renewing an appropriate cycle time.
In another preferred form of the apparatus of the present invention, the apparatus further comprises a switch device for allowing or prohibiting execution of the time compensation in the time controlled step or steps. The provision of the switch device permits optionally selecting execution of the time compensation in the injection molding cycle. Therefore, the time compensation can be prohibited in the case where a precise control of time is required in the time controlled step, for example, whereby the adverse effect of the time compensation on the step can be avoided as needed. In the case where the time compensation is executed at the plurality of time controlled steps, a plurality of the switch devices may be provided for the respective time controlled steps.